1. Field of the Invention
The present invention relates to the synchronization of digital signals in communication links in general. More specifically, the invention relates to the synchronization of digital signals of packets and frames of packets in half-duplex communication links especially by means of optical fibers.
Half-duplex transmission of communication signals is an interesting solution for optical fiber telecommunications networks. In fact, this solution enables a single optical fiber to be used for two transmission directions and for several subscriber communications, thereby ensuring optimum efficiency of the fiber's very high capacity.
In a half-duplex communication, e.g. between a telephone exchange and a subscriber installation, the link between the exchange and the subscriber installation is not permanent. This requires synchronization to be recovered at the beginning of each communication period. In order to achieve good listening comfort for the communication, synchronization recovery, also known as frame alignment recovery in ISDN type telecommunications networks, should be performed extremely quickly, usually in less than 10 milliseconds.
2. Description of the Prior Art
The known frame alignment recovery devices are usually designed for duplex type bidirectional communications. In these devices, frame alignment is declared acquired, when the communication is set up, after a specific frame alignment pattern has been successively received several times; this entails a relatively long frame alignment recovery time. Such devices are therefore not suited to speedy recovery of frame alignment in a half-duplex communication.